Chemical coagulants have long been used in the treatment of water and waste water to induce flocculation of particles suspended in the raw water to be treated. This aggregation of suspended particles allows for more efficient sedimentation and/or filtering downstream. For best results, the initial mixing of the chemical coagulant with the raw water should occur as rapidly as possible to form a homogenized mixture within one or two seconds.
The principal objective of this rapid or flash mixing is to ensure a homogeneous coagulation by completely uniform dispersion of the coagulant throughout the water. In this way, the coagulant can make contact with the maximum number of suspended particles prior to the completion of hydrolysis, enabling intermediate complexes to destabilize the suspended particles initiating aggregation. This chemistry of destabilization sets some of the requirements for efficient rapid mixing.
Chemical coagulants should be dispersed in an unblended stream of raw water. Dispersing chemicals into a blended or partially blended stream (backmixing) can lead to poor destabilization of a fraction of the particles because some might have insufficient surface coverage while others might have too extensive surface coverage by adsorbed chemical species. This wastes chemicals and results in less effective floc formation for a given amount of a coagulant.
Stagnation time, defined as the amount of time that elapses from the addition of a coagulant to the start of mixing, should be reduced for most effective coagulation.
From a mechanical point of view, a rapid mixing device should be simple, practical and relatively inexpensive and should not create appreciable head losses.
Through the years, in attempting to meet these chemical and mechanical requirements, many devices have been employed to provide the rapid mixing needed for chemical dispersion. These include the weir, the Parshall Flume, rapid mixing chambers equipped with mechanical rotary mixing devices such as propellers or turbines and in-line blenders. More recently, hydraulic diffusion flash mixing has been used as a method providing rapid mixing without appreciable head losses and lower operating and maintenance costs than mechanical methods. This method also provides more efficient rapid mixing with reductions of 20 to 30 percent in chemical coagulant consumption over mechanical methods.
Generally hydraulic diffusion flash mixing operates by drawing off a portion of the raw water to be treated into a carrying water loop. The chemical coagulant to be dispersed is added to this drawn off portion. The mixture of raw water and coagulant is then injected into the remainder of the raw water through a diffuser. A pump in the carrying water loop provides the pressure for injection.
Usually the diffuser is a radial jet diffuser which injects the raw water and coagulant mixture perpendicular to the flow direction of the remaining raw water from several nozzles equally spaced about the circumference of a tube placed in the center of the pipe carrying the remaining raw water. Radial injection can also occur by injection perpendicular to the flow direction from nozzles equally spaced about the pipe periphery. In theory, this alternative reduces head losses, but is more difficult to construct, so central injection is preferred.
Sometimes the diffuser is a conical jet diffuser which injects the raw water and coagulant mixture parallel to the flow direction of the remaining raw water through a single nozzle, directed either upstream or downstream with the flow, located in the center of the pipe carrying the remaining raw water. Both of these options are versions of the central injection scheme. Because flow through a conical nozzle requires more power than the convergent nozzle used in the radial jet, and because the water leaving the conical nozzle does not flow entirely perpendicular to the direction of the raw water, thus causing a degree of backmixing, the radial jet diffuser is preferred over these options.
Problems have developed with hydraulic flash diffusion mixing in some applications. Where hardness exists in the raw water to be treated, addition of coagulant in the carrying water loop has led to clogging of the diffuser nozzles. This clogging requires periodic plant shutdowns to clean the diffuser, resulting in greatly increased operating and maintenance costs.
A hydraulic diffusion flash mixing system in which coagulant is directly introduced into the raw water flow is disclosed in U.S. Pat. No. 4,869,595 to Lang. In this system, raw water flows in the main pipe, and a portion of this water is diverted and reintroduced into the main pipe by a narrow auxiliary pipe. The auxiliary pipe's outlet is formed by numerous small nozzles around the periphery of the auxiliary pipe for injecting raw water perpendicular to the main raw water flow direction. A coagulant feed pipe leads to a manifold positioned around the auxiliary pipe adjacent the nozzles. The manifold has its own nozzles which surround the auxiliary pipe and inject coagulant in the direction of the main raw water flow, i.e., perpendicular to the direction of the auxiliary raw water flow so that the coagulant flow and auxiliary water flow mix and at the same time mix with the main water flow. However, the numerous injection nozzles create a relatively complex structure and are not immune from clogging due to particulate impurities in the coagulant. This is so because of the relatively small volume of the coagulant flow in relation to raw water flow, on the order of a million times less, and because the coagulant nozzles must be small enough so that the headloss through them is high enough to ensure that coagulant is properly dispensed through each coagulant nozzle.